Semiconductor wafers are slices composed of semiconductor materials such as, for example, elemental semiconductors (silicon, germanium), compound semiconductors (for example composed of an element of the third main group of the periodic system such as aluminum, gallium or indium and an element of the fifth main group of the periodic system such as nitrogen, phosphorus or arsenic) or compounds thereof (for example Si1-xGex, 0<x<1). They are required, in particular, as basic material for electronic components and have to meet stringent requirements with regard to flatness, cleanness and lack of defects.
Flat slices composed of other materials are required for other applications, for example glass slices as substrates for producing magnetic memory disks or slices composed of sapphire as a support for manufacturing optoelectronic components.
Such slices composed of semiconductor material or some other material are sliced from a rod consisting of the respective material. In particular, a chip-removing machining method such as slicing lapping is appropriate for slicing the slices. Chip removal or chipping is understood to mean, according to DIN 8580, mechanical machining methods in which material is brought to the desired form by removing excess material in the form of chips. In this case, the term chip denotes a particle detached from the workpiece.
According to DIN 8589, lapping is chipping using loose grain distributed in a liquid or paste (lapping slurry) as abrasive, which is guided on a normally shape-bearing counterpiece (lapping tool) with the cutting paths of the individual grains being as far as possible non-directional. The material removal is effected by brittle-erosive separation of the material cohesion via the formation of micro cracks at the penetration location of the lapping grain and spalling of small material particles. Lapping is based on a three-body interaction between workpiece, lapping grain and lapping tool. Lapping is characterized by the fact that the tool carrier (lapping disk, lapping wire) does not contain hard substances which come into engagement with the material in chipping fashion.
Grains composed of diamond, silicon carbide, boron carbide, boron nitride, silicon nitride, zirconium oxide, silicon dioxide, aluminum oxide, chromium oxide, titanium nitride, tungsten carbide, titanium carbide, vanadium carbide and many others, and also mixtures thereof, are used as loose lapping substances supplied during slicing lapping.
Diamond, silicon carbide and aluminum oxide, in particular silicon carbide, are important as lapping substances when slicing semiconductor wafers.
In the case of single slicing lapping, exactly one slice is sliced from the workpiece per cut; in the case of multiple slicing lapping, a multiplicity of slices are sliced simultaneously per cut. Multiple slicing lapping can be carried out using a wire which is multiply diverted via rolls, such that it comes multiply into engagement with the workpiece. This is then referred to as single-wire multiple slicing lapping. Alternatively, methods are known in which a multiplicity of individual wires, which are fixedly braced in a frame like strings of a harp, work through the workpiece. This is then correspondingly referred to as multi-wire multiple slicing lapping. The present invention relates generally to the slicing of a multiplicity of slices from arbitrarily shaped workpieces composed of arbitrary materials which can be machined in chipping fashion. The invention relates particularly to the slicing of a multiplicity of slices from prismatically shaped workpieces having rectangular, hexagonal or octagonal base surfaces or of cylinders composed of glass, sapphire or semiconductor material. Single-wire multiple slicing lapping is described more thoroughly below. This is also referred to in shortened designation as slurry wire sawing (SWMS “slurry multi-wire slicing”).
An apparatus for single-wire multiple slicing lapping (“slurry wire saw”) comprises as essential apparatus features wire, at least two wire guide rolls arranged horizontally and parallel with respect to one another, a take-off and a take-up spool, an apparatus for pre-tensioning a wire in the wire longitudinal direction, a feed apparatus, by means of which the workpiece can be fed perpendicularly to the axes of the wire guide rolls toward the plane spanned by the axes, and an apparatus for applying an abrasive in the form of a slurry of loose hard substances in a carrier liquid. The wire guide rolls are cylindrical and mounted rotatably about their longitudinal axes. Their lateral surfaces have a multiplicity of grooves running concentrically about the axis and largely equidistantly with respect to one another.
In the case of slurry wire sawing, the wire is guided under tension by means of the grooves spirally multiply via the wire guide rolls such that individual wire sections become situated in parallel fashion and form a web. By rotating the wire guide rolls in the same sense, the wire is unwound from the take-off spool and wound onto the take-up spool. In this case, the wire sections of the web respectively move parallel to one another in the wire longitudinal direction. In order to simplify the explanation, the workpiece is assumed hereinafter to be a cylindrical rod composed of semiconductor material (semiconductor rod). Said semiconductor rod is adhesively bonded at its lateral surface to an axially running strip composed of a sacrificial material (sawing strip), for example composed of glass or graphite, and is clamped by means of the latter and with its workpiece axis parallel to the axes of the wire guide rolls in the feed apparatus.
By slowly feeding the rod parallel to the perpendicular of the sawing web toward the web, the workpiece comes into contact with the web by that section of its lateral surface which is situated opposite the sawing strip, and a force builds up in the wire transverse direction between tool (web) and workpiece. As a result of the relative movement between workpiece and web on account of the sawing wire moved through the apparatus, material removal is effected under pressure and with addition of the abrasive. By maintaining the wire transverse tension by means of further continuous feeding of the rod, the wire web works through the entire cross section of the workpiece, and a multiplicity of slices are obtained simultaneously.
Single-wire multiple slicing lapping can be effected with a direction of movement of the wire sections of the web that is constant over the entire cut, or with reversal of the direction of movement. In this case, cutting with continual reversal of the wire direction is of particular importance since specific disadvantages for the achieved flatness and front/rear side parallelism of the slices obtained are avoided by the reversal of direction. Asymmetries between the entry side of the wire sections and the exit side of the wire sections can be averaged out by the reversal of direction and thus are partly compensated for, and the wire consumption can be reduced by the reversal of direction.
The reversal of direction of the wire run corresponding to the pilgrim step method (“pilgrim step motion”, “wire reciprocation”) comprises a first movement of the wire in a first wire longitudinal direction by a first length and a second movement of the wire in a second direction, which is exactly opposite to the first direction, by a second length, wherein the second length is chosen to be less than the first length. For each pilgrim step, overall a wire length corresponding to the sum of both lengths thereby runs through the workpiece, while the wire section which comes into cutting engagement with the workpiece in this case moves further only by a magnitude corresponding to the difference between the two lengths from the take-off toward the take-up spool. In the pilgrim step method, therefore, the wire is utilized multiply in a ratio of the sum to the difference of the two lengths.
After working through the entire cross section of the workpiece, the wire web reaches the sawing strip adhesively bonded onto the workpiece. The further feed is stopped and the now multiply severed workpiece is withdrawn again from the sawing web by reversal of the feed direction. The workpiece has now been separated into a multiplicity of slices which adhere by part of their circumference to the half-severed sawing strip equidistantly and parallel to one another and perpendicular to the workpiece axis. By chemical, thermal or mechanical release of the adhesive bond, the slices are separated and supplied to a further application-dependent subsequent processing.
The slurry wire sawing and an apparatus suitable for slicing semiconductor wafers are described for example in EP 0 798 091 A2.
The flatness of the sliced slices that can be achieved by wire slicing lapping is impaired by a multiplicity of effects. These include effects related to the kinematics of the wire, the supply and distribution of the abrasive in the sawing gap, the wear of the wire and the sawing grain. Thermal processes have a particularly great influence on the cutting result. It is known from DE 101 22 628 that chipping work and friction processes bring about a heat input into the workpiece that leads to an axial relative movement between the workpiece and the wire sections. In a cylindrical workpiece, the length with which the sawing wire is in engagement with the workpiece changes with the cutting progress. The heat input and thus the axial relative movement between the workpiece and the wire sections consequently changes slowly (quasi-statically) with time. When cutting into and when cutting out of the workpiece, abrupt changes in the engagement lengths are present, and the cutting rate that results given a constant wire transverse tension is particularly high. Therefore, a particularly great axial relative displacement between workpiece and web occurs during cutting in and out, with the result that all slices of the sawing cut acquire a flatness deviation curved out of the ideal sawing plane substantially in the same sense and to the same extent. This flatness deviation, referred to as sawing-in and/or -out undulation, is particularly harmful since it has a long wavelength (several centimeters) and in this case impairs the parallelism of the front and rear sides of the slices (thickness homogeneity) only to a small extent. Since the semiconductor wafers exhibit largely elastic behavior in the range of centimeters, (or longer), the sawing-in and/or -out undulation cannot be removed, or can be removed only inadequately, by the material removal accomplished by the subsequent processing steps.
Such undulatory slices are unsuitable for demanding applications. In the case of slicing lapping of large, and particularly in the case of slicing lapping of very large, workpieces into slices, these undesirable thermally governed defects are particularly pronounced. Workpieces having a large diameter are those whose area-equal circle upon projection of a cross section along the principal axis with the smallest moment of inertia has a diameter (equivalent diameter) of greater than or equal to 300 mm; workpieces having a very large diameter are those having an equivalent diameter of greater than or equal to 450 mm.
JP 10180750 describes a method in which the temperature of the abrasive supplied to the sawing gap by spraying from above onto the sawing web is adapted in a closed control loop of temperature regulation of the abrasive and temperature measurement and temporally variable heating is thus counteracted.
DE 101 22 628 B4 describes a method in which the entire part—situated above the wire web—of the lateral surface of the rod is flushed with coolant that is temperature-regulated in a manner dependent on time and cutting progress, and the rod is thus temperature-regulated.
EP 0 798 091 A2 describes methods in which the volumetric flow rate of the abrasive supplied, the viscosity and the feed rate of the rod to the sawing web are altered in a manner dependent on the cutting progress.
Finally, U.S. Pat. No. 7,959,491 B2 describes a method in which the temperature of the slicing lapping agent supplied is increased continuously steadily, but in a manner dependent on the instantaneous position of the web in the rod, over the entire cutting progress from sawing in until sawing out and partial compensation of the thermal effects is thus performed in a manner dependent on the web position in the rod.
The slicing gaps hidden in the rod can be observed if the workpiece is pellucid or transparent at least in a certain spectral range. Thermal observations of the slicing zone on a rod composed of silicon which is transparent in the infrared spectral range showed, by means of a thermal imaging camera, that the heat input into the sawing gap and over the length of the sawing gap is not effected uniformly. In particular, it was observed that the temperature in the slicing gap increases with the engagement length from the wire entry toward the wire exit. The hottest point is reached shortly before wire exit; directly at the surface upon wire exit, the temperature decreases again somewhat, probably via thermal emission and air convection at that surface of the rod which is close to this point. The heating along the wire engagement thus takes place in a very complicated way.
In the slicing gap of a workpiece composed of silicon, a temperature increase of more than 20° C. is observed over the wire engagement length, and just approximately 5° C. in the mass of the surrounding silicon volume that has not yet been cut. The heat gradient over the sawing gap reverses on a short timescale (a few seconds) if the direction of wire movement reverses during slicing lapping preferably carried out in the pilgrim step method. These dynamic temperature fluctuations are considerable, take place for a short period of time with the frequency of the pilgrim step and far exceed the averaged workpiece temperature that varies only slowly over the cutting progress.
The known methods only compensate for this slow quasi-static temperature change. They are unsuitable for compensating for the rapid and much higher temperature changes and the effects thereof, in particular the resulting undulation of the sliced slices.